(1) Field of the Invention
The present invention relates to a support sheet for photographic paper. More particularly, the present invention relates to a support sheet useful for photographic paper having an enhanced resistance to curling, especially twist-curling.
(2) Description of the Related Art
A waterproof support sheet for photographic paper, comprising a substrate paper sheet and two coating layers formed on the two surfaces of the substrate paper sheet, which comprises a waterproof polyolefin resin, is widely used to accelerate a developing process of the photographic paper. The photographic paper is produced by coating a photosensitive emulsion containing gelatine, as a bonding material, on a waterproof-surface of the support sheet.
As well known in the art, the conventional waterproof support sheet is disadvantageous in that, when a photo print prepared from a photographic paper comprising the conventional waterproof support sheet is placed on a flat plane, both edge portions of the print curve upward and are raised from the flat plane so that the print is curled.
Usually, the amount of curling of the print sheet is measured and represented as follows.
A print sheet having a width of 82 mm and a length of 114 mm is placed on a flat plane at a relative humidity (RH) of 50%, at room temperature. The heights of four end points of the corners of the print sheet from the flat plane are measured, and the amount of curl of the print sheet is represented by an average of the heights of the four corner points.
Usually, the conventional print sheets exhibit an amount of curl of 1 to 5 mm at 50% RH at room temperature.
British Patent No. 1269802 and Japanese Examined Patent Publication (Kokoku) No. 48-9963 (1973) disclose a method of preventing this curling. In this method, two surfaces of a substrate sheet are coated so that one coating layer has a different weight, thickness, and/or polymer density than the other coating layer. The above-mentioned difference between the two coating layers, itself causes a curl in the resultant coated sheet. Also, when a photographic emulsion layer containing gelatine is applied to a surface of the coated sheet, the emulsion layer creates curl in the resultant photographic paper sheet. Therefore, in a resultant print sheet, the curling derived from the coating layers on the substrate sheet can be counterbalanced by the curling derived from the emulsion layer.
Nevertheless, it has been found that, when the above-mentioned balanced print sheet is left in a low humidity atmosphere, for example, at a relative humidity (RH) of 30% or less at a temperature of 30.degree. C or more, which atmosphere may be encountered immediately after a drying step in an automatic developing process, even the balanced print sheet frequently generates an amount of curl of 5 to 30 mm. When the direction of the axis of curl created in a print sheet is parallel to the direction along which the print sheet is forwarded in an automatic developing machine, the curl of the print sheet does not create undesirable obstacles, for example, jamming and stacking, resulting in difficulty in the automatic developing operation, unless the amount of curl is at an unusual high level of 20 mm or more at 30% RH at 30.degree. C.
But when the direction of the axis of curl is at a intersects the direction along which the print sheet is forwarded in the automatic developing machine, the print sheet is twisted during the automatic developing process and the resultant twisted curl of the print sheet frequently not only generates difficulties in the automatic developing operation but also significantly decreases the commercial value of the print sheet.
Generally, it is considered that the cause of the generation of curl in a photographic paper sheet resides in a difference in shrinkage between the gelatine-containing photographic emulsion layer formed on a surface of a support sheet and the support sheet when the photographic paper sheet is dried.
Namely, when the print sheet is dried after development or placed in a low humidity atmosphere, the gelatine-containing photographic emulsion layer on a surface of the photographic paper sheet is dried at a relatively high drying rate and thus the volume of the emulsion layer is rapidly reduced. But, in the print sheet, the shrinkage and reduction in volume of the substrate paper sheet, having two hydrophobic polyolefin coating layers coated on two surfaces thereof are gradual.
The above-mentioned phenomena result in the creation of differences in the shrinkage and reduction of volume of the emulsion layer and the substrate paper sheet, and thus the print sheet is curled in such a manner that the gelatine-containing photographic emulsion layer, which is subject to a larger shrinking force than that of the other layers, forms the inside surface of the curled sheet.
Generally, it is found that, when the print sheet is subjected to the developing and drying operations, the intensity of the resultant curl or twisted curl of the print sheet is relatively small at a drying temperature of about 50.degree. C to about 60.degree. C, but becomes significantly large at a drying temperature of about 70.degree. C to about 80.degree. C. The reason for the influence of the drying temperature on the curling of the print sheet is not absolutely clear, but it is assumed that, at a high drying temperature of 70.degree. C or more, the flexibility and softness of the support sheet, especially the polyolefin resin coating layers, is increased, in other words, the resistance of the support sheet to deformation becomes low, and thus the support sheet is more easily curled by the shrinking force of the gelatine-containing photographic emulsion layer.
Recently, there has been a strong demand for automatic developing machines be able to be operated at an increased high speed, and to meet this demand, there is a trend toward raising the drying temperature. But, the high temperature, high rate drying operation for the print sheet in the automatic developing machine further promotes the creation of curl or twisted curl in the print sheet.
Accordingly, there is a strong demand for an improved support sheet having a reduced curl-forming property.